(a) Field of the Invention
The invention relates to a diversity receiver, and particularly to a diversity receiver for an orthogonal frequency division multiplexing (OFDM) system.
(b) Description of the Related Art
A typical orthogonal frequency division multiplexing (OFDM) system, one kind of multi-carrier system (MCS), includes a transceiver and a receiver. The OFDM system can provide considerably high bandwidth utilization efficiency to result in a high data transmission rate, because all sub carriers transmitted from the transceiver are orthogonal to each other and are individually modulated. Besides, the orthogonality between different sub-carriers may effectively diminish multi-path fading. Therefore, the OFDM system is suitable for various wireless communication systems, such as wireless local area network (WLAN) and fourth-generation (4G) mobile communication.
However, in a time-variant channel, interference between different sub-carriers and rapid channel fading (frequency-selective fading) may seriously degrade system performance and cause a high bit error rate to result in an error floor, even channel estimation and equalization treatments are indeed performed by the receiver.
Hence, a diversity reception technique is proposed to solve the aforesaid problems, which is widely used in various applications and particularly for mobile reception. Typically, a diversity receiver includes at least two antennas and their respective signal processing units for subsequent treatments. The two antennas are separately provided for receiving different versions of the same transmitted signal, and the signal processing units are used to combine input signals from different transmission paths.
Referring to FIG. 1, a conventional diversity receiver 10 includes two branches 11 and 12 having similar components, a combination/selection unit 1a, and a Viterbi decoder 1b. The branch 11 includes a Fourier transform circuit 111, a channel estimator 112, a channel equalizer 113, and a soft demapper 114. Further, the branch 12 includes a Fourier transform circuit 121, a channel estimator 122, a channel equalizer 123, and a soft demapper 124.
When a transmitter (not shown) transmits a first version input signal II(n,k) regarding a nth symbol and a kth sub-carrier (n and k are positive integers) to the diversity receiver 10, the Fourier transform circuit 111 receives the first version input signal II,(n,k) via an antenna and transforms it into a first frequency-domain signal Y1(n,k). On the other hand, when the transmitter transmits a second version input signal I2(n,k) regarding a nth symbol and of kth sub-carrier to the diversity receiver 10, the Fourier transform circuit 121 receives the second version input signal I2(n,k) via an antenna and transforms it into a second frequency-domain signal Y2(n,k). The mathematical models for the frequency-domain signals Y1(n,k) and Y2(n,k) are given by the following equation:Y1(n,k)=H1(n,k)S1(n,k)+V1(n,k)Y2(n,k)=H2(n,k)S2(n,k)+V2(n,k)  (1.1)where H1(n,k) and H2(n,k) are respective channel frequency responses of the first and second versions of input signals I1(n,k) and I2(n,k), S1(n,k) and S2(n,k) are transmission data transmitted by the transmitter, and V1(n,k) and V2(n,k) are additive white Gaussian noises (AWGN). The relationship between additive white Gaussian noises of different channels is given by:σv12≠σv22 which indicates the signal variants of the branch 11 are different to that of the branch 12, i.e. the background noises of the branch 11 and that of the branch 12 are different to each other. However, it should be noted the above relationship does not mean the noises V1(n,k) and V2(n,k) are completely unrelated.
The channel estimator 112 fetches the first frequency-domain signal Y1(n,k) and evaluates the estimate value of the channel frequency response H1(n,k) according to a reference signal (such as a pilot signal) contained in the first frequency-domain signal Y1(n,k). Then, the estimate value of the channel frequency response H1(n,k) is fed to the channel equalizer 113. Similarly, the channel estimator 122 outputs the estimate value of the channel frequency response H2(n,k) to the channel equalizer 123. The channel equalizer 113 receives the first frequency-domain signal Y1(n,k) and generates a signal M1(n,k) according to the estimate value of the channel frequency response H1(n,k). Similarly, in the second branch 12, the channel equalizer 123 generates another signal M2(n,k) through the same treatments. The signals M1(n,k) and M2(n,k) are given by:M1(n,k)=|H1(n,k)|2S1(n,k)÷H1*(n,k)V1(n,k)M2(n,k)=|H2(n,k)|2S2(n,k)+H2*(n,k)V2(n,k)  (1.2)where H1*(n,k) and H2*(n,k) are respective complex conjugates of H1(n,k) and H2(n,k).
Next, the signal M1(n,k) is divided by |H1(n,k)|2 by means of a divider in the channel equalizer 113 of the branch 11 to generate a first equalized signal Eo1(n,k). Similarly, a second equalized signal Eo2(n,k) is generated by the same division operation performed by the channel equalizer 123 of the branch 12. Thus, we obtain:Eo1(n,k)=S1(n,k)÷{(H1*(n,k)V1(n,k))/|H1(n,k)|2}Eo2(n,k)=S2(n,k)+{(H2*(n,k)V2(n,k))/|H2(n,k)|2}  (1.3)
Further, the values of the divisors, namely |H1(n,k)|2 and |H2(n,k)|2, are fed to the combination/selection unit 1a and serve as reference information for the Viterbi decoder 1b. 
Typically, the noise term in Equation 1.3, i.e. {(H1*(n,k) V1(n,k))/|H1(n,k)|2} or {(H2*(n,k) V2(n,k))/|H2(n,k)|2}, is so small as to be neglected compared to the transmission data S1(n,k) and S2(n,k). Hence, the transmission data S1(n,k) and S2(n,k) can be extracted after equalization and then respectively transmitted to the soft demappers 114 and 124. The soft demappers 114 and 124 perform symbol demapping on them to respectively generate demapped signals Sf1(n,k) and Sf2(n,k) that are fed to the combination/selection unit 1a. 
Finally, the combination/selection unit la perform either combination or selection on the demapped signals Sf1(n,k) and Sf2(n,k) and the channel frequency responses H1(n,k) and H2(n,k) according to their response qualities to generate a decode signal E. The decode signal E is transmitted to the Viterbi decoder 1b to generate decoded data O.
During the equalization performed by the conventional diversity receiver 10, a complicated division algorithm as well as a divider is required to provide the divisor values of |H1(n,k)|2 and |H2(n,k)|2 for the Viterbi decoder 1b as reference decoding information. However, this may cause complexity in demodulation operations performed by the diversity receiver and may increase manufacturing costs due to the need of the divider.
Moreover, in the conventional design, since the channel weights of different branches set by their respective channel equalizers are equal to each other, the Viterbi decoder 1b can be provided with only channel information but without background noise information about each channel. Therefore, the decoding performance of the Viterbi decoder 1b is difficult to be improved.